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The present invention relates to motor-operated jacks for raising or lifting vehicles to desired heights and, more particularly, to a motor-operated jack consisting of a power transmission unit and a lifting unit, and designed to reduce an undesired power consumption due to the frictional resistance formed at the junction of the engaging threads of the lifting unit, and have a means for reliably making the spiral lifting ram be lifted prior to the secondary screw during an operation of the jack, and have a means for allowing a user to control the moving stroke of the lifting unit as desired.
In the prior art, jacks used for raising or lifting the vehicles are classified into several types: screw-type jacks, rack-type jacks, pneumatic or hydraulic jacks, and motor-operated jacks. In recent years, several types of motor-operated jacks have been proposed and widely used. One example of such conventional motor-operated jacks is referred to U.S. Pat. No. 4,609,179.
The construction of the motor-operated jack disclosed in the above U.S. patent document is shown in FIG. 1. As shown in the drawing, the jack is designed such that the output power of a drive motor b is transmitted to the lifting ram 1 through a plurality of power transmission gears d and e so as to move the ram 1 in a vertical direction. However, the output power of the drive motor b is not sufficient to effectively raise or lift a heavy vehicle, and so an unexpected breakage of the drive motor or the power transmission gears may occur during an operation of the jack. Such an unexpected breakage of the drive motor or the power-transmission gears may cause safety hazards while raising or lifting a vehicle. In addition, it is almost impossible to move the ram of the above motor-operated jack using the electric power of a vehicle.
Another example of conventional motor-operated jacks may be referred to Korean U.M. Publication No. 92-5683. This motor-operated jack is designed such that a safety unit, in addition to a reduction gear unit having two sets of planetary gears, is provided at the output side of a drive motor, and is somewhat smoothly operable by electricity supplied from the cigar lighter provided on the dashboard. Since the jack has the safety unit, it is possible to almost completely prevent unexpected occurrence of safety hazards caused by overload during an operation of the jack. Therefore, this jack does not have any problem in its power transmission unit. However, this jack is problematic in its lifting unit as follows.
First, as shown in FIG. 2, the above jack undesirably consumes excessive power due to frictional resistance produced by the first engagement of the external thread 103a of the primary screw 103 with the internal thread 104a of the secondary screw 104 and the second engagement of the external thread 104b of the secondary screw 104 with the internal thread 105a of the spiral lifting ram 105.
Second, the secondary screw 104 may be undesirably raised along with the spiral lifting ram 105 at the same time during a rotating action of the primary screw 103 even though the spiral lifting ram 105 is needed to be lifted prior to the secondary screw 104 during such a rotating action of the primary screw 103. Furthermore, the secondary screw 104 and the spiral lifting ram 105 are undesirably raised at the same time during such a rotating action of the primary screw 103 when the frictional resistance produced at the junction of the external thread 104b of the secondary screw 104 and the internal thread 105a of the spiral lifting ram 105 is higher than that of the junction of the external thread 103a of the primary screw 103 with the internal thread 104a of the secondary screw 104. Particularly when the secondary screw 104 is undesirably raised along with the spiral lifting ram 105 at the same time during such a rotating action of the primary screw 103, only the primary screw 103 is left at the lower portion of the jack to bear the load applied from the vehicle to the jack. In such a case, the primary screw 103 may be broken due to overload, and causes safety hazards during the operation of the jack.
Third, the jack is designed to raise or lift a vehicle, and so the jack is inevitably limited in the length of the lifting ram in an effort to accomplish the compactness of the jack. It is sometimes necessary for the jacks to lift a vehicle to a great height. However, the above jack does not have any means for allowing a user to control the moving stroke of the lifting unit as desired, and so the jack cannot raise or lift a vehicle to a desired great height when necessary.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a motor-operated jack for vehicles, which is designed to reduce an undesired power consumption due to the frictional resistance formed at the junction of the engaging threads of the lifting unit, and having a means for reliably making the spiral lifting ram be lifted prior to the secondary screw during an operation of the jack, and have a means for allowing a user to control the moving stroke of the lifting unit as desired.
In order to accomplish the above object, the present invention provides a motor-operated jack for vehicles, comprising a lifting unit consisting of: a spiral lifting ram having an internal thread limitedly formed at a lower portion of the spiral lifting ram, thus reducing its frictional resistance; a pin held in a horizontal groove formed at the top end of a primary screw, the pin being slightly projected into two longitudinal holes symmetrically formed at the top portion of a secondary screw; a coil spring, a spring cover and a snap ring sequentially laid on the top of the pin such that the snap ring is seated in an annular groove formed on the inner surface of the top wall of the secondary screw, thus allowing the coil spring to bias the pin, the second screw being thus prevented from being raised along with the spiral lifting ram; and a support ram having a top support and an external thread, and movably fitted in a gap between the secondary screw and the spiral lifting ram while engaging with an internal thread of the spiral lifting ram at the external thread, thus allowing a user to control the moving stroke of the lifting unit.